Powertrain Control Modules (PCMs), used in vehicles to control engines and power transmission systems, must not interfere with surrounding electronic components (radiated emissions) and must also be capable of operating normally when subjected to certain levels of electromagnetic radiation (radiated immunity).
In some electronic control modules, a capacitor is placed in series between a copper ground plane of a printed circuit board (PCB) and a case ground, or metallic module housing. The metallic module housing is then mechanically and electrically joined to the vehicle's chassis via machine screws, brackets, or bolts, providing a substantially low impedance path for ground return currents to flow through. This design will provide some immunity to external electromagnetic interference (EMI), but may not necessarily quell radiated emissions. To achieve the goals of minimizing radiated emissions, while preserving radiated immunity, improvements are needed to provide suppression of radiated emissions.
Rakouth et al. describe a circuit for damping out radiated noise resulting from resonances in a controller module. (Rakouth, Comstock, and Cammin, Characteristics of AC Grounding in an automotive ECU, 2001 IEEE Intl Symposium on EMC). The Rakouth et al. circuit is composed of a 10 nF capacitor in series with a resistor, all of which is interposed between the case ground and controller B-line. In one embodiment, Centola, Bruno, et al, describe a method for electrically coupling PCBs' ground planes along their edges to the surrounding chassis ground in patent application 20020071265 as a means of reducing radiated emissions from a PCB. Patent Application 20010014963 describes a method for designing decoupling circuits centered around carefully chosen capacitors. Moreover, U.S. Pat. No. 6,016,089 discusses the use of common mode filtration capacitors that resiliently connect the ground plane of a PCB to a module case, or housing.
Virtually all PCMs equipped with Motorola Oak processors or similar type of processor have experienced electromagnetic compatibility (EMC) issues. Radiated emission (RE) levels from these modules typically exceeded acceptable levels.
Many experiments were attempted to reduce RE over the years. The most effective countermeasure involved modifying the case-grounding scheme of PCMs. In the past, the PCB was grounded, at a single point, to the case through a capacitor, with values typically ranging from 10 nF to 400 nF. The capacitor was put in place to reduce the risk of electrostatic discharge damage and radiated immunity issues.
Therefore, there is a need for an new and improved method and system for providing electromagnetic radiated immunity and reduced electromagnetic emissions. The new and improved method and system should minimize radiated emissions while preserving radiated immunity.